1. Field of the Invention
The present invention relates to techniques to improve durability of electric double layer capacitors.
2. Background Art
Electric double layer capacitors may be used in a wide range of temperatures and have high power density. In order to improve the energy density of such capacitors, it is necessary to increase working voltage. As an electrolytic solution, a nonaqueous electrolytic solution (organic electrolytic solution) which mainly consists of at least one of an open-chain carbonate and a cyclic carbonate as a solvent and a quaternary ammonium salt as a supporting salt, can be widely used. In the case in which the voltage is increased in an electrolytic solution containing at least one of an open-chain carbonate and a cyclic carbonate as a main solvent, the electrolytic solution gradually decomposes, internal pressure in a capacitor housing is increased by generated gas, and many problems occur.
Furthermore, in a capacitor using alkali-activated carbon, when the electrolytic solution in which solvent consists of at least one of an open-chain carbonate and a cyclic carbonate is used at high working voltage under high temperature conditions, the amount of gas generated by solvent decomposition is increased, and this is therefore not preferable. That is, it is difficult for such an electrolytic solution to be used at high working voltage under high temperature conditions because decomposition of the solvent is promoted and gas, such as carbon dioxide gas, is generated.
In addition, considering improvement in conductivity, change ratio of capacitance, etc., various solvents or mixed solvents have been proposed as an electrolytic solution for capacitors. In regard to suitability in practical use of electrolytic solutions for capacitors, there are various requirements for physical characteristics, such as low-temperature characteristics, solubility of salt (dielectric constant), safety, decomposability of the electrolytic solution, boiling point, cost, etc., and as an electrolytic solution for capacitors which satisfies these requirements, open-chain carbonates and cyclic carbonates are widely used.
As an electrode material, since an alkali-activation method can produce activated carbon having high capacity, a method in which activated carbon is produced by alkali-activating easily graphitizable carbon material, is known (Japanese Unexamined Patent Application Publication No. Hei 09-275042). However, in a capacitor using alkali-activated carbon, the amount of gas generated by solvent decomposition is greater than one in a capacitor using steam-activated carbon, and many problems occur when cell structures form. In particular, in the case in which a carbonate type solvent is used at high working voltage under high temperature conditions, the solvent is decomposed and CO2 is generated.
As a technique which reduces the amount of decomposed gas such as carbon dioxide gas, etc., a method which uses solvent containing γ-butyrolactone or γ-valerolactone (Japanese Unexamined Patent Application Publication No. 2001-217150), a method which uses an organic electrolytic solution containing fluorobenzene (Japanese Unexamined Patent Application Publication No. 2004-6803), and a method which uses a nonaqueous electrolytic solution containing diphenyl (Japanese Unexamined Patent Application Publication No. 2004-146610), have been suggested.
However, in the case in which alkali-activated carbon consisting of graphite material is used, the amount of the generated gas cannot be controlled sufficiently by the above techniques.